The portrait, in keeping with the Duchess's wishes, does not portray her official side but the woman known to her family and friends.

Emsley's efforts have been praised for their "informality", but have also been criticised for being "pretty ordinary".

Kate's response to the work after seeing it privately with her husband the Duke of Cambridge was to say: "It's just amazing, I thought it was brilliant," while William described it as "absolutely beautiful".

Emsley was commissioned by the National Portrait Gallery and was chosen by its director Sandy Nairne following a selection process which involved the Duchess.

Richard Stone, Britain's most prolific royal portrait artist, said the work had captured Kate's evident warmth and approachability. He said of the portrait: "I liked it, very much so. So often with official portraits they can be rather stiff and starchy, but this has a lovely informality about it, and a warmth to it."

But art critic Waldemar Januszczak, who writes for The Sunday Times, said he was "disappointed" by the portrait and that there was a lack of sparkle in Kate's eyes.

Emsley, 65, a Glasgow-born painter who grew up in South Africa but has lived in the UK since 1996, said after the unveiling in London: "I was delighted to get the commission and then after that it began to sink in to me how important this would be.

"A person whose image is so pervasive, for an artist it is really difficult to go beyond that and find something which is original - you have to rely on your technique and your artistic instincts to do that and I hope I've succeeded."